Yes I'm not sure having a Contax 645 body around to change the aperture is going to be realistic. I need to find out how to stop down to f8 or f11 on a dumb adapter.
I realize I'm way late to this thread, but for the benefit of others looking for solutions to address this issue, I thought I'd respond regardless.
Instead of carrying around a C645 body, I use a Fringer adapter with a Contax 645 NAM-1 adapter mounted on a tiny Sony A5100 I bought used for $125 solely for this purpose ... works great! And without the Fringer adapter, it also works with my Contax N lenses, too!
To make it easier to change apertures (as well as minimize the amount of handling of the lenses required to do so, because I'm afraid of --
gulp! -- accidentally dropping them), I mounted a small tab on my FrankenKamera and mounted the A5100 / Fringer combo on that:
I'm not sure whether other Sony to C645 smart adapters will also work with the NAM-1 -- I know the original Kipon smart adapter I had would not -- but it might be worth investigating, because the Fringer adapter is quite a bit more expensive than the other options.